my @pids;
{
    open(my $pids_fh, '<', $pids_qfn)
        or die("Can't open PIDS file \"$pids_qfn\": $!\n");
    push @pids, /\|(\d+)$/ while <$pids_fh>;
}

my $pids_pat =
    map qr/$_/,
    join '|',
    #map quotemeta,  # We're only dealing with digits
    @pids;

open(my $fids_fh, '<', $fids_qfn)
    or die("Can't open FIDS file \"$fids_qfn\": $!\n");

while (<$fids_fh>) {
    print if /^$pids_pat;/;
}

[download]

If you need extra speed and your Perl is older than 5.10, change 

my $pids_pat =
    map qr/$_/,
    join '|',
    #map quotemeta,  # We're only dealing with digits
    @pids;

[download]
to 
use Regexp::List qw( );
my $pids_pat = Regexp::List->new()->list2re(@pids);

[download]

5.10 already does the optimisation Regexp::List does.

In reply to Re^3: My code sucks, please help me understand why. (regex) by ikegami
in thread My code sucks, please help me understand why. by mirage4d

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.